While virtually all magnetic recording media now in use have magnetizable layers comprising magnetizable particles dispersed in organic binder, the amount of information that can be recorded in any such medium is reaching the theoretical limit. It has long been known that information can be recorded more compactly on metallic thin-film magnetic recording media such as the medium disclosed in U.S. Pat. No. 2,644,787 (Bonn et al.). That patent disclosed a method of electrodepositing onto an electrically conductive substrate a magnetizable layer from an aqueous plating bath having a pH in the range of 2 to 6 and including as essential elements nickel ions in a concentration in the range of 0.2 to 1.7N, cobalt ions in the range of 0.2 to 1.0N and hypophosphite ions in the range of 0.04 to 0.2N. Current densities ranged from 10 to 200 amperes per square foot (11 to 215 mA/cm.sup.2) and bath temperatures were on the order of 45 to 55.degree. C. with an upper limit of about 80.degree. C. Higher temperatures may have produced spontaneous reduction throughout the bath. A specific product was reported to have a coercivity of approximately 810 oersteds and a remanence of the order of 10,000 gauss from a bath containing 0.84N in nickel, 0.84N in cobalt, 0.145N in hypophosphite ions, and ammonium chloride in 1.9 molar concentration, with a current density of 50 amperes per square foot (54 mA/cm.sup.2) at 50.degree. C.
U.S. Pat. No. 3,578,571 (McQuaid et al.) disclosed an effort to reduce the cost of thin-film magnetic recording media by reducing the high concentration of components in the plating bath. Improved coercivity levels and particularly squareness ratios were said to be achieved, although the reported coercivities were within the range of the Bonn patent. McQuaid's bath temperatures were between 32.degree. and 66.degree. C., and current densities were between 5 and 10 amps/dm.sup.2 (50 and 100 mA/cm.sup.2).
It can be assumed that the magnetic values of the Bonn and McQuaid media were measured in the planes of the coatings. Media made according to the example of each of those patents would exhibit mixed in-plane and perpendicular magnetic anisotropy.
Sato et al., "Magnetic Properties of Electro-deposited Co-Ni-P Recording Films", Denki Kagaku, Vol. 35, No. 2, pp. 111-15, 1967, (a publication in the Japanese language), also concerns electrodeposition of magnetizable thin-film coatings from a bath of cobalt, nickel and hypophosphite ions. The Sato publication reports that when the current density was kept low (0.5 A/dm.sup.2 or 5 mA/cm.sup.2), the magnetizable coating had perpendicular anisotropy. Somewhat higher current densities resulted in a distributed orientation, and at a current density of 2.0 A/dm.sup.2 or 20 mA/cm.sup.2, the coating had in-plane anisotropy. The only bath temperature mentioned was 25.degree. C. The pH was 6.0.
Because the crystallite c-axes of electro-deposited coatings having in-plane anisotropy tend to be oriented in all planar directions, it is recognized that perpendicular anisotropy should enable more compact and efficient recording of information, since all crystallite c-axes can be oriented in the same direction. Also, magnetic recording media having perpendicular anisotropy exhibit a significantly reduced demagnetizing effect at high recording densities.
Recent efforts to develop thin-film magnetic recording media having perpendicular anisotropy have concentrated on cobalt-chromium coatings. See, for example, U.S. Pat. No. 4,210,946 (Iwasaki). No recent publications have been found concerning cobalt-nickel-hypophosphite electrodeposition, perhaps because of Sato's teaching of very low current densities to obtain perpendicular anisotropy, hence economically impractical rates of electrodeposition.